Probably the most important physical property of metallurgical coke is its strength or ability to withstand breakage and abrasion during handling and during its use in the blast furnace. The standard test to evaluate coke strength is the stability index test (ASTM D3402) which involves tumbling coke of selected size in a standard drum rotated for a specific time at a specific rate. The stability index is reported as the percentage of coke remaining on a one inch screen when the coke is screened after tumbling. In general, a stability index of at least about 50 to 60 is required for acceptable strength metallurgical coke. Preferably, the stability index should be at least about 50 to 55 for a small or medium blast furnace and at least about 58 to 60 for a large blast furnace.
Petrographic evaluation of coking coal is relied upon to predict the quality of the coke that can be expected, particularly the stability index of the coke. The basis for this reliance is evident from the findings of prior investigators, as reported in the technical literature, that the coking properties of coal depend on the coal's rank and its inert content. Coal rank is a measure of the degree of alteration of the coal-forming plants that has occurred because of geological factors. This degree of alteration is also affected by the types of original plant materials. Inerts are components of coal that do not react beneficially during the coking process. The inerts may be either inorganic minerals, which are non-coking, or organic macerals that have been severely altered and rendered poorly coking or non-coking by geological or environmental factors.
Coal rank may be measured chemically in terms of dry mineral matter-free volatile matter or may be measured petrographically in terms of vitrinite reflectance, in accordance with standard ASTM tests. It has been found that coal rank, as determined by these tests, generally correlates with coke stability. However, even if the rank of the coal indicates it should produce high coke stability and the mineral or ash content is normal, the actual stability of the coke may be low because of high organic inerts or because of atypical behavior of the non-inert portion of the coal. Furthermore, a coal that has been used successfully in the past to make coke with an acceptable stability index may sometimes undergo an apparent deterioration resulting in coke of inferior strength.
In the preparation of coal for coking, various coal cleaning or washing techniques are customarily used to remove high ash coal particles, solid foreign matter such as rock and slate, and other free impurities. The coal cleaning processes in current use are predominantly of the sink-float type in which cleaning is possible because of the difference in specific gravity between the free impurities or refuse (typically 1.8 to 6.0) and the coal particles (typically 1.25 to 1.55). A liquid separating medium or parting liquid is used which has an intermediate specific gravity such that the heavier refuse particles sink and the lighter coal particles float. In some cases the separating medium is an aqueous suspension of ground solids, such as sand, magnetite or barite. In other cases so-called high density or heavy liquids are used, such as aqueous calcium chloride solutions. Various halogenated hydrocarbons have also been proposed as high density liquids but have been used for the most part in coal washing laboratories. Examples of prior art patents showing the use of halogenated hydrocarbon liquids for coal cleaning are: Keenan Pat. No. 2,109,234; Alexander et al Pat. No. 2,150,899; Foulke et al Pat. No. 2,150,917; Alexander et al Pat. No. 2,151,578; Tveter Pat. No. 3,348,675; Dessau Pat. No. 4,076,505; Smith et al Pat. No. 4,173,530; and Smith et al Pat. No. 4,244,699.
Coal cleaning results in a reduction of the ash and sulfur content of the coal and in most instances improves the coking properties of the coal because of the lowering of the inorganic mineral content. For example, it is known that reduction of the ash content of coal by appropriate cleaning usually improves the stability index of the coke obtained from the coal. In the case of certain coals, however, reduction of the ash content by the usual cleaning methods does not result in sufficient improvement of the stability index of the resultant coke to make the coal acceptable for coking purposes.
Accordingly, a need has existed for a method of treating certain potentially useful coking coals to insure the production of coke having an acceptable stability index.